Mystery Man
by smile-lifegoeson
Summary: AU. Eren finds out that Mikasa has a boy in her life. He drags Armin off to find out who!


It's been a while since I posted something that wasn't Fairy Tail. I liked it. This fic is really random and probably isn't that great. But it was fun to write! I needed something silly to get me writing again. I got a new laptop and wanted to post something but wasn't sure what. So I went with this. Hehe. Just something goofy I came up with.

Sorry if the characters are a little OOC. I've never wrote for Attack on Titan before. But I hope you like it!

 _Attack on Titan_ and it's characters belong to Hajime Isayama

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"Eren, why are we at Mikasa's so early in the morning?" Armin asked as he and his best friend went up the stairs to his other best friend's apartment. She wasn't home at the moment but Eren was insistent on going there.

"Because I need to check on something," Eren dug around the flower pot next to door number 7.

"And we couldn't do this while she was home?"

"Yes!" Eren found the key hidden in the dirt, "No way man. She'd kill me."

Armin watched his friend go into his step-sister's apartment, knowing all too well how Mikasa valued her privacy.

"Hmm, I guess in her bedroom," Eren walked farther into the space leaving Armin by the door.

"She really will kill you if you sneak around her bedroom. What do you need to check anyway?" Armin refused to do any snooping.

"COME HERE ARMIN! I FOUND SOMETHING!"

Armin sighed, and even though he knew he would regret it, he followed Eren's call, "What is it?"

Eren was sitting in the floor in the bathroom connected to Mikasa's bedroom. He was pointing at the garbage can.

"Uh," Armin took a glance in it, "so you needed to know if she threw away her trash?"

"No! Look at that!" Eren forcibly moved Armin's head in one direction and pointed again.

"Q-tips? Everyone uses q-tips."

"The condom Armin! There is a USED condom!"

Armin had seen it, but had hoped it wasn't what Eren was referring to.

"She's being safe. That's good to know," Armin stood, crossing his arms as he watched steam come out of Eren's ears.

"Dude! Don't you get it?! Mikasa has a boyfriend!"

"Yes. And?"

"Doesn't this surprise you?!" Eren walked back into her bedroom, looking for more clues.

"No. I already knew that."

Armin knew that was the wrong thing to say when Eren turned around, his eyes filled with hurt and a ridiculous amount of betrayal.

"What?! Who is it?! Why didn't you tell me?!"

"I don't know who it is. She just said she was seeing someone. And it's not my place to tell you, she said she wanted to make sure it was serious before telling you. How did you figure it out anyway?"

"I called her a few weeks ago and asked if she wanted to get dinner and she said she was working all night. But I saw her car at that fancy Italian place not even an hour later! I've been finding all kinds of clues here lately," Eren growled, "I can't believe she kept this from me!"

"Why?" Armin sat down in Mikasa's living room recliner, as Eren was now looking for clues in the kitchen, "You've never been great at keeping secrets. And anytime Mikasa did tell you something about a guy she liked you'd hunt them down and threaten them."

"Well duh," Eren peaked in the pantry, "she's my sister. I gotta take care of her."

"She can take care of herself. And telling someone that Mikasa actually likes that they need to stay away from her is the opposite of taking care of her," Armin sighed, Eren took being a brother too far sometimes.

"LOOK AT THIS!" Eren came running into the living room.

Armin looked at the item now resting between Eren's hands, "Tea? It's Mikasa's, Eren. She likes tea."

"Not this kind! I've seen her turn this down so much! It's gotta be her boy toy's!"

Armin groaned, "Please don't call him that."

Eren had already run back into the kitchen, "I found something else!"

Armin came into the kitchen, seeing Eren looking in the dish washer.

"Look at this mug!"

"What about it?" Armin took the mug that said _'I hate Mondays'_.

"That's not Mikasa's. I helped her move, I know what kind of coffee mugs she uses and that's not one of them!" Eren jumped up, grinning from ear to ear.

"No, it's Levi's."

"Huh? Levi?"

"Yes, it's his see?" Armin held the mug up so Eren could see Levi's name on the bottom.

"Damn, I thought that was a lead," Eren deflated a bit.

Armin put the mug aside. Levi Ackerman was a few years older than the trio, being a senior in high school when they were freshman. Eren idolized Levi, especially after he was saved by the smaller man from a brutal beating. Armin respected him, he was in a lot of the same academic clubs that Armin picked. Mikasa hated his guts. To save Eren from the beating, he actually beat Eren himself. She was unable to forgive him for several months.

It took a Christmas party at Erwin Smith's home for her to finally forgive him. They had played dirty Santa, Levi having drawn Mikasa's name. He got her a new set of boxing gloves, something that she had really needed but never told anyone about. He had noticed that she needed new ones when he was working out on his own. She then learned the truth about everything he did for Eren, and finally forgave him. They started to train together after that.

Now, Levi and Mikasa worked side by side at Stohess General Hospital. Levi is the hospital's lead diagnostician. Mikasa works directly with Levi, but is also a top surgeon.

"I bet the tea is his too," Armin pointed out. Levi and Mikasa often had late night talking about patients, it wasn't an odd occurrence to find them at the others home.

"Damn it!" Eren fell into one of Mikasa's dinner chairs, "there went all my clues."

"Sorry, Eren," Armin pated his back, "let's go. I'm getting hungry and I really don't want to mess up her apartment."

"Right!" Eren jumped up and ran to the door, "To the hospital!"

"Hospital?" Armin, double checked the kitchen and living room for anything out of order, a few thing here and there caught his eye before he locked the door behind him.

"Yep. We're going to have lunch with Mikasa. Maybe if I spend some time with my dear sister we can figure out more about this guy she's sleeping with."

"Since when do you call her dear sister? In school you hated it when acted like your sister," the boys went back down the stairs, "and she's not just sleeping with this guy. It's an actual relationship Eren."

"She's always been my dear sister Mikasa," the look on his face said he agreed more with Armin's statement, "and until she tells me or I meet this guy, there is no relationship to me to know about."

Sighing, Armin followed Eren down the street leading to the hospital, _'I hope whoever this guy is knows how protective everyone is of Mikasa.'_

Walking inside the giant hospital, Eren took in all the sights before wandering to the elevator.

"Did you let Mikasa know we were coming?" Armin asked, hitting the button for the fourth floor.

"Yeah, as much as I'd like to catch her in the act I didn't want to see anything too graphic."

"Catch her in what?!" Armin's hair covered his face as he turned quickly at Eren's comment.

"The act," Eren held up a finger, "for one, she's always here. So whoever she is seeing works here too. Two, since she's here all the time that means that her and this guy probably sneak off to do who knows what kind of kinky stuff when they can. Three-"

"OK, OK," Armin put his hand over Eren's mouth, "You made your point. But Mikasa is a very professional person. She probably wouldn't jeopardize her job just for sex."

The door pinged open, thankfully no one heard Armin's last word.

"You're probably right. But-" Eren gasped incredibly loud, "WHAT IF IT'S ERWIN?!"

"Erwin?" Armin turned the corner to head to Mikasa's office, "Why do you think that?"

"It's so clear! She's always here, she's been going to some really fancy high-class restaurants, and she got a raise last week!"

"Just because she got a raise last week does not mean she is dating the Dean of Medicine."

"What about the fancy food?" Eren smirked.

"Mikasa makes enough money that she can go by herself," Armin glared at him, Eren knew how much Mikasa made, after all she paid half of his bills almost every other month.

"Fine."

"Eren? Armin?"

The two turned around to find Jean Kirstein and Reiner Braun, classmates of theirs. The two worked as surgical assistants.

"What's up man," Eren fist bumped Reiner, "Been a while."

"It has. Why haven't you stopped by for poker night, too scared I'll see you in your panties again?" the burly man laughed as Eren's cheeks darkened.

"Sure he is; he's got a terrible poker face. Even worse that Bert's," Jean laughed, enjoying his frenemies pain.

"I have to agree," Armin shyly looked away when Eren glared at him.

"I do not! I've just been busy! I'll be there next time and I'll show you!" Eren shouted, getting several dirty looks from the nurse's station.

"Good. You and Marco can be the waiters as the real men play," Reiner smirked, then the three laughed as Eren ground his teeth and his face bright red.

"Don't you two need to be working?"

"Sorry sir," Jean squeaked out, "we're on our way now."

Levi looked exhausted as he glared at the two assistants holding two cups of coffee. Reiner and Jean waved to Eren and Armin then went down another hall way.

"What are you two brats doing here?" Levi walked past them, heading to his shared office with Mikasa.

"Eating lunch with Mikasa," Eren ran to hold the door open for the doctor, "and actually, I think you can help me.

"Eren no," Armin groaned. As much as Levi liked Eren, he seriously doubted if the man would help out in this situation. He'd probably tell Mikasa, since he refuses to be on her bad side ever again.

"Help with what?" Levi placed the cups of coffee on the large table that was covered in papers and folders and x-rays.

"I think Mikasa has a boyfriend. And I want to find out who," Eren's eyes looked like a child's who just entered toy land.

Armin watched Levi carefully, the man gave off subtle clues, but you had to watch every moment or you'd miss it. Unfortunately, Armin didn't see anything except annoyance creep into his tired eyes.

"Why don't you just ask her?" Levi sat heavily into his rolling chair, propping his feet up on the table.

"She didn't even tell me she was in a relationship, I found a condom in her bathroom and I know she's been on a few dates lately," Eren sat across from Levi, having a feeling that his answer was close.

Armin's eyes widened a bit, he saw Levi's lip twitch, as if he was fighting a smile.

"I don't give a damn who she sleeps with. So you better just ask her yourself," Levi sipped his coffee as he read over a large file.

"Ask me what?" Mikasa herself walked into the room, looking just as exhausted as Levi. She was holding a large stack of files, earning a groan from Levi.

"More? What the hell does Erwin want with all this?"

"I'm just doing what he told me," she placed the files on the table before turning to Eren, "what do you need to ask me?"

"NOTHING!" He stood quickly, "I told you Armin and I would come eat lunch with you."

"Oh, right," she rubbed a hand over her neck, "now's not the best time. We're having to go over all these files from last year and we're nowhere near done. Maybe dinner sometime this week."

Eren grumbled, "You'll be too busy with your new boy toy to even remember me."

"Eren," Armin hissed.

"What?!" Mikasa screamed. Levi sputtered into his coffee.

"Oh uh," Eren quickly made an exit.

"What.. boy toy?!" She turned to Armin, a million questions in her eyes.

"He knows you have a boyfriend. He's made it his mission to find out who it is."

"Oh, he's going to be the death of me," Mikasa ran off to catch her step-brother and set him straight. Armin carefully watched Levi, waiting for a reaction.

A scowl into his coffee was all he needed to know.

"So," he started slowly, sitting in Eren's discarded chair, "how long have you an Mikasa being seeing each other?"

At first he wasn't sure Levi was going to answer, then he breathed in deeply before saying, "Too damn long for her not to say anything to the brat."

Armin smiled, so he was right all along. Eren probably didn't catch it, but the tea and mug in Mikasa's apartment had been there for a while. And while it was true they spent many nights working on cases together, Mikasa would never buy tea for Levi and keep it at her apartment for those rare instances. He also saw a few more things in the apartment that belonged to Levi.

"Why haven't you told anyone yet?" Armin had a feeling their jobs kept them from saying anything, but it could be more.

"Erwin knows," Levi glared off to the side, "encouraged it actually. She wants to be more serious before saying anything. And I don't like people knowing my business, so it works for both of us."

"Then why are you telling me?"

Levi leaned back again, "It'll be funny as hell when the brat does find out and you'll have known all along."

Armin sunk into his chair. It'll be funny to Levi, but for him he might as well pretend he didn't know a thing.

"Ow! Let go already!" Armin and Levi could hear Eren complain, then Mikasa lecture him about this and that.

"Oh yeah, before they come back in here," Levi said, getting Armin's attention, "tell that brat to stay out of her apartment when she's not there. I'd really hate to have to kick his ass when the woman finally lets me in her bed."

Armin flushed at the mental image.

"Geez, I'm just trying to do something nice," Eren winced as she rubbed his sore ear.

"You can still be nice and not do something like break into my apartment."

"Yeah, yeah. Let's go, Armin. I'm starving," Eren waved goodbye to Levi and drug his blonde friend out of the room.

"Did she tell you anything?" Armin had a much better poker face than Eren did, and was able to act as if nothing changed.

"NO! She said that yeah she had a boyfriend but she wasn't ready to say who and man it sucks! I'd never hide a relationship from her! I mean, we grew up together and did.." Eren went on to say more but Armin had glanced back to the office to see the pair beyond the glass wall. His eyes widened and a smile graced his face when he saw Mikasa lean down and kiss Levi. At least they weren't hiding it.

"Hey I know!" Eren shook Armin's shoulder, "What if it's Reiner? Or Jean?!"

"Oh no.."

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Well not my best work. But it was fun :) I may write another AOT fic, I love Mikasa and Levi as a couple so maybe something a little more about them. Anyway, hope you enjoyed! And if you are a follower of any of my FT fic's, hopefully I'll get to update soon. I've just been real busy here lately! Follow my instagram to find out why haha

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